Accurate, reliable analyte detection in physiological fluids, such as whole blood, blood plasma, serum and urine, is becoming increasingly important for clinical diagnosis and management of diseases and conditions in humans and animals. For example, diabetes is typically diagnosed by determining whether the patient has abnormally high blood glucose levels, and once diagnosed, management of the disease often involves maintaining blood glucose levels within an acceptable range by dietary adjustments or medication. Studies have shown that careful control of blood glucose levels can significantly reduce the incidence of serious complications of diabetes such as vision loss, limb amputation and kidney failure.
In order to control blood glucose levels, some diabetics are required to test their blood glucose levels several times daily. Portable blood glucose meters are available which permit a patient to determine blood glucose levels quickly with a reasonable degree of accuracy. In general, these devices utilize disposable test strips that include chemicals that produce a color or electrochemical change, when a drop of a patient's blood is applied to the strip, in proportion to the amount of blood glucose present in the blood. For strips that change color, the strip bearing the patient's blood is inserted into the meter and the color change is measured using an optical reflectance system within the meter. The meter then generates a digital read-out corresponding to the concentration of glucose in the blood.
In order to obtain an accurate measurement of an analyte it is necessary to determine whether a spectrophotometer or reflectance meter is operating properly by, for example, measuring the amount of an analyte, such as glucose, that is present in a control solution that contains a predetermined (i.e., known) amount of the analyte. One problem encountered with control solutions for reflectance meters is that the dye used to produce the color change in response to the amount of analyte present tends to coalesce with other dye molecules. Dye coalescence increases with the length of time the control solution is on the test strip. This causes the analyte concentration reading to drift which leads to an inaccurate reading of analyte concentration. Thus, improvement in the stability of analyte concentration readings in control solutions is desirable.
In addition, optical reflectance meters provide accurate results only if the test strip is inserted into the meter properly and only if there is enough sample on a test strip. Since blood absorbs light at wavelengths around 940 nm, many meters are designed to detect absorbance of light at this wavelength. For an example of this type of meter see U.S. Pat. No. 5,605,837, the entire teachings of which are incorporated by reference. If the meter detects insufficient absorbance of light at this wavelength, an error warning informs the user that not enough blood was applied to the strip. In general, control solutions for such reflectance meters contain a substance that mimics the 940 nm blood absorbance of light so that the meter will inform the user when too little control solution has been applied to a test strip. In some control solutions, a suspension of particles, such as a suspension of carbon particles as is found in black india ink, has been used for this purpose. However, suspensions of carbon particles may precipitate out of solution to such an extent that the meter no longer recognizes that an appropriate amount of the control solution has been applied to a test strip. Therefore, it would be desirable to have an alternative means for simulating the absorption of light by blood at 940 nm in a control solution.